When we changed from the A1 to the A2 we never shot A2 ammo in an A1, but did not worry about shooting A1 in an A2....I know, I'm giving my age away

I have noticed that many modern civilian AR's are labeled in 223/5.56

That's true. My Bushmaster is labeled that way. With some of the newer guns on the market, I would think that one could get away with firing a 5.56 Nato out of a .223 Rem, but that over time it would cause undue wear and tear on the weapon and possibly render it unsafe. Weapons are an investment and it would be a shame to degrade the value simply because you fired the wrong type of ammo when the right type is readily available.

On another note, even within the 5.56 Nato there are many different specs. When I was in Afghanistan, we tried to resupply a British unit with ammo and apparently our 5.56 ball was not the same as what they use in the SA80 assault rifle. I never had an opportunity to learn more about the differences between their ammo and ours, but perhaps someone who is familiar with both can share his/her thoughts?

Here's an interesting conundrum. I have a DPMS heavy 20" barreled .223. Says so on the barrel. However, the lower says .223- 5.56NM. I emailed DPMS and asked if I can shoot 5.56NATO in it. They said an emphatic "NO". I have Black Hills ammo and on it's box it says 5.56. No NATO. They don't have an email address. I did look at their web site and my ammo is listed as .223. No listing for 5.56NATO or even 5.56 anywhere. Since I already thought it was ok when I bought them, I shot some with no trouble. Again, I'm not saying it's ok, but I do think B.H. ammo , even though the box says 5.56, is actually new .223 match ammo. Anybody got any thoughts?

Check out Rifleshooter mag. In the March/April issue they have a write up on the 223/5.56 issue. The main issues are the throat length, the slope at the origin of the rifling, and the start pressure for each round.
I'd rather not dig a bolt carrier(or bolt in a bolt out of my teeth after a few careless moments, so I bought an ar in 5.56 nato.
Most ammo labeled either 5.56 or 223 is at lower specs, so many of you that think you are shooting nato spec ammo really aren't.

I do agree that chambers vary but that would apply to all guns and calibers. If this were so critical you would see data for 5.56 AND .223 but you don't.
Here is some of the reason for the confusion. They test differently thus two sets of pressure data.

Wikipedia:

While there is a myth that 5.56 NATO cases are thicker and hence have less capacity than commercial .223 cases, this has been shown to be false. Each brand of case and each manufacturing lot has a slightly different case capacity; 5.56 NATO and .223 commercial cases tend to have nearly identical case capacity when measured using the water test.[6] The NATO specification allows a higher chamber pressure. NATO EPVAT test barrels made for 5.56mm NATO measure chamber pressure at the case mouth, as opposed to the location used by the United States civil standards organization SAAMI. The piezoelectric sensors or transducers NATO and SAAMI use to conduct the actual pressure measurements also differ. This difference in measurement method accounts for upwards of 137.9 megapascals (20,000 psi) difference in pressure measurements. This means the NATO EPVAT maximum service pressure of 430 megapascals (62,000 psi) for 5.56mm NATO, is reduced by SAAMI to 379.21 megapascals (55,000 psi) for .223 Remington.[7] In contrast to SAAMI, the other main civil standards organization C.I.P. defines the maximum service and proof test pressures of the .223 Remington cartridge equal to the 5.56mm NATO.
__________

[QUOTE=venatic;655066]I do agree that chambers vary but that would apply to all guns and calibers. If this were so critical you would see data for 5.56 AND .223 but you don't.
Here is some of the reason for the confusion. They test differently thus two sets of pressure data.
Yes, the nato is at cip rating, and the 223 is sammi, but you miss the point entirely. The chamber dimensions of the 5.56 chamber allow heavier charges at safe pressure. When you shove this ammo in a rifle with a tight throat( a 223), you will be over pressure. I'm getting to see why my brother won't buy a used rifle.